GB2186595A

GB2186595A - Cast iron roll with hard outer surface

Info

Publication number: GB2186595A
Application number: GB08702116A
Authority: GB
Inventors: Gerard Barbezat; Heinz Luscher
Original assignee: Sulzer AG; Gebrueder Sulzer AG
Current assignee: Sulzer AG
Priority date: 1986-02-14
Filing date: 1987-01-30
Publication date: 1987-08-19
Also published as: US4771524A; CH667285A5; GB8702116D0; GB2186595B; DE3640131A1; DE3640131C2

Description

GB2186595A 1 SPECIFICATION carbidic structure, the roll having a minimum

diameter of 200 mm and a minimum wall A roll having a hard outer surface thickness of 20 mm, the carbidic structure of the running surface having a nominal depth, This invention relates to a roll having a hard 70 from which the actual depth at no part of the outer surface or skin, more particularly for a running surface of the roll departs by more paper-making machine, and made of gray cast than 10% of the nominal depth, of at least 1 iron having an outer running surface with a mm and its cementite dendrites having a max carbide structure, the roll having a minimum imum length of 100 pm and a maximum thick- diameter of 200 mm and a minimum wall 75 ness of 20 jum.

thickness of 20 mm, and to a process for the According to a second aspect of the inven production of such rolls. tion, in a process for the production of a roll The term---roils-is intended in the present according to the first aspect of the invention, case to denote solid rolls and hollow rolls. the roll is first teemed into a sand mould, They can be used in papermaking machines 80 then preheated from the cast state, the pre more particularly for smoothing, drying and heated running surface is melted locally by ra creeping the web of paper. If necessary they pid heating and in progressive steps to the can additionally be heated or cooled for these intended nominal depth by rapid heating to uses. The linear force pattern along the nip above the liquidus temperature, and then soli between the roll and, for example, another roll 85 dified by an immediately subsequent rapid can usually be controlled along the roll length cooling below the Ar, point of the iron-carbon by variable internal or external forces. diagram, whereafter the roll is cooled to ambi All percentages indicating concentrations of ent temperature.

compositions of material are to be understood The rapid heating of the preheated running as being in weight %. 90 surface to at least the liquidus temperature of Rolls or roll shells as hereinbefore described the cast iron used-i.e., to at least approxi experience heavy wear. They are therefore mately 1200'C-and the subsequent rapid produced as chilled castings in which a chill solidification and cooling to below the Ar, po casting surface layer with a carbide structure int of the iron-carbon diagram-i.e., to below produced by chilling of the mould arises on 95 730'C-produces on the running surface a the outside of a lamellar graphite cast iron fine carbidic and more particularly a ledeburitic core which solidifies as gray cast iron because structure whose cementite dendrites do not of relatively slow cooling. exceed the claimed maximum dimensions; the Because cooling rates differ, three discrete point is that the treatment dissolves the car- zones having different structures arise: 100 bon in the melted zone, the carbon subse (a) The gray cast iron zone in which the quently crystallizing as iron carbide because of carbon has separated out mainly as graphite, the high solidification and cooling rates. To namely lamellar graphite; the structure in this wards the interior of the roll body, the carbi zone is perlitic; dic layer is followed by a heat-influenced zone (b) A transitional or mottled zone of white 105which can be considered to have a fine-perlitic and gray cast iron in which the proportion of bainitic quenched and tempered structure, and white (carbidic) cast iron decreases continu- the thickness of the latter zone corresponds at ously towards the inside and the separation as least approximately to the thickness of the lamellar graphite increases correspondingly; carbidic layer preceding it.

the extent and nature of this transitional zone 110 The required remelt depth can be achieved vary within wide limits and escape arbitrary only if wall thickness is at least 5 times the control; remelt depth, otherwise sufficiently rapid cool (c) The chilled zone in which the carbon has ing and solidification of the melted depth zone been completely bound as carbide; this zone becomes uncertain.

has a coarse structure containing cementite 115 Since the depth of the melted zone in the dendrites whose length is mainly 200 lim and remelting of the surface can be determined whose thickness is mainly more than 50 lim. and observed relatively accurately, the required Dendrite orientation is relatively random; the nominal depth can be ensured over the entire coarse structure impairs abrasion resistance. surface to the required accuracy and with the Since defined chill cast layers are difficult to 120 required uniformity.

achieve in chill casting, rolls of the kind speci- To reduce the risk of cracking more particu fied cannot be produced from spheroidal larly in the transition zone between the layer graphite or vermicular graphite cast iron. of the running surface and the roll body and It is an object of the invention to increase core unaffected by remelting, the maximum the abrasion resistance of the running surfaces 125 nominal depth of the carbidic structure is, with of such rolls by reducing the grain sizes of the advantage, 8 mm. This maximum nominal carbidic structure. depth makes economic sense too since it can According to one aspect of the invention, a be achieved with cooling conditions that are roll having a hard outer skin and made of gray not particularly expensive.

cast iron has an outer running surface with a 130 Very advantageously, the new roll is made 2 GB2186595A 2 of spheroidal graphite cast iron and has the following composition (in weight Ni 0.1-3 Cu 0.1-2 c 2.3-3.8 M0 0.1-1 Si 1-3 70 Sn 0.01-0.2 Mn 0.1-1 Cr 0.01-0.4 P max 0.08 B 0.01-0.1.

S max 0.01 Mg 0.03-0.08 Since the roll surface requires machining Fe Remainder; 75 after remelt hardening, the minimum depth of the melted and carbidically solidified running- the point is that the modulus of elasticity, surface zone must before such machining be which is important for rigidity, is the same approximately 1 mm more than the required over the whole roll cross-section including the nominal depth.

carbidic layer and is, for example, approxi- 80 Advantageously too, in the production pro mately 160 000 to 170 000 Nlmm2. In con- cess described preheat temperatures of from trast to this, in the rolls forming part of the 450 to 60WC and heating rates of at most prior art hereinbefore described the modulus 1OWC/h are maintained and/or the cooling to of elasticity in the carbidic layer is equally ambient temperature after remelting proceeds high, but it drops continuously in the mottled 85 at a maximum rate of WC/h; conveniently, zone in dependence upon graphite content and the latter cooling treatment can be given in a is only about 100 000 to 120 000 N/MM2 in furnace.

the gray zone. Also, spheroidal graphite cast The energy source for the melting operation iron has the further advantage of increased can be more particularly electric arcs, in which fatigue strength over conventional gray cast 90 event welding torches, more particularly those iron. which have a tungsten electrode and operate, Of course the rolls can be made of vermicu- for example, a an energy density of 2-4 lar graphite cast iron or lamellar graphite cast kW/CM2, are particularly suitable because of iron, in which case the following composition their relatively simple handling and for eco (in weight %) has been found advantageous 95 nomic reasons. Since the melting proceeds for rolls having segregations of vermicular advantageously in a protective gas atmos graphite: phere, the energy source used is preferably in the form of TIG (tungsten inert gas) welding c 2.8-3.6 torches. However, the melting can be per P max 0.06 100 formed by means of laser or electron beams.

si 1-3 The rapid cooling to temperatures below Mn 0.1-1 73WC proceeds in still air; as previously S max 0.06 stated, the minimum wall thickness specified Mg 0.02-0.04 ensures that heat is removed fast enough. The Fe Remainder. 105 simplest way of achieving the stepwise ad vance of local melting is by a rotating and/or For roll bodies made of conventional cast longitudinal relative movement between the iron-i.e., of lamellar graphite cast iron-the torch and the preheated roll; it has been found following composition (in weight %) has convenient in this case if the roll is rotated proved advantageous: 110 around the stationary torch while advancing axially in steps.

c 2.8-3.6 Conveniently too, to achieve a flaw-free run P 0.01-0.5 ning surface having a carbidically solidified S max 0.1 structure, the roll is treated mechanically to Si 0.5-3 115 remove casting skin before it is given the Mn 0.1-1 treatmnt according to the invention. After the Fe Remainder. latter treatment the running surface is finish machined conventionally, as previously stated, Other roll properties such as tensile strength carbidic running surfaces of from 1 to 8 mm and fatigue strength can be improved in all 120 then arising.

three kinds of graphite segregations if the cast The invention will be described in greater iron also contains at least one of the following detail hereinafter with reference to an alloy elements (in weight Example.

A roll shell of 400 m outer diameter and 70 125 mm wall thickness, made of spheroidal graph ite cast iron having the following chemical composition (in weight 3 GB2186595A 3 which is made of vermicular graphite cast iron c 3.4 and has the following composition (in weigh si 2.4 p 0.2 S 0.1 70 C 2.8-3.6 Mn 0.2 P max 0.06 Cu 1 si 1-3 Mg 0.04 Mn 0.1-1 Fe Remainder S max 0.06 75 Mg 0.01-0.04 is cast as a sand casting in a sand mould; in Fe Remainder.

the cast state it is initially preheated by a gas torch to a preheat temperature of 50WC while 5. A roll as claimed in claim 1 or claim 2 slowly rotating. Fixedly disposed opposite the which is made of lamellar graphite cast iron shell outer periphery is a TIG welding torch 80 and has the following composition (in weight having a tungsten electrode of 3.2 mm dia meter. The roll surface moves past the elec- trode at a speed of approximately 15 cm/min, c 2.8-3.6 a voltage U of 20.5 V between the electrode p 0.01-0.5 and the workpiece giving rise to an arc in 85 S max 0.1 which a current 1 of approximately 200 A si 0.5-3 flows. Mn 0.2-1 A helium flow of 7 1/min from the torch Fe Remainder.

maintains the melt zone of the torch arc in a protective gas atmosphere. Immediately after 90 6.A roll as claimed in claim 3 or claim 4 the torch the molten zone cools immediately in which the cast iron contains at least one of below the Ar, point of the iron-carbon dia- the following alloy elements (in weight gram.

After completion of remelting the roll is Ni 0.1-3 heated in a furnace preheated to 50WC, then 95 Cu 0.1-2 cooled slowly at a maximum cooling rate of Mo 0.1-1 WC/h. Sn 0.01-0.2 In the present example this leads to a re- Cr 0.01-0.4 melt depth of approximately 6 mm. B 0.01-0.1 100

Claims

CLAIMS 7. A roll as claimed in any of the preced

1. A roll having a hard outer skin and ing claims which is a papermaking machine made of gray cast iron having an outer run- roll.

ning surface with a carbidic structure, the roll 8. A process for the production of a roll having a minimum diameter of 200 mm and a 105 as claimed in any of claims 1 to 7 in which minimum wall thickness of 20 mm, the carbi- the roll is first teemed into a sand mould, dic structure of the running surface having a then preheated from the cast state, the pre nominal depth, from which the actual depth at heated running surface is melted locally by ra no part of the running surface of the roll de- pid heating and in progressive steps to the parts by more than 10% of the nominal 110 intended nominal depth by rapid heating to depth, of at least 1 mm and its cementite above the liquids temperature, and then solidi dendrites having a maximum length of 100 lim fied by an immediately subsequent rapid cool and a maximum thickness of 20 pm. ing below the AR, point of the iron- carbon 2. A roll as claimed in claim 1 in which the diagram, whereafter the roll is cooled to ambi- nominal depth of the carbidic structure is not 115 ent temperature.

greater than 8 mm. 9. A process as claimed in claim 8 in 3. A roll as claimed in claim 1 or claim 2 which the roll is preheated to a temperature which is made of spheroidal graphite cast iron between 45WC and 60WC.

and has the following composition (in weight 10. A process as claimed in claim 8 or %): 120 claim 9 in which the heating rate of the pre heating is at most 1OWC/h.

c 2.3-3.8 11. A process as claimed in claim 8 or si 1-3 claim 9 or claim 10 in which the roll is cooled Mn 0.1-1 at a maximum cooling rate of WC/h.

P max 0.08 125 12. A process as claimed in any of claims S max 0.01 8 to 11 in which the running surface is melted Mg 0.03-0.08 in a protective gas atmosphere.

Fe Remainder. 13. A process as claimed in any of claims 8 to 12 in which the roll is cooled in a fur- 4. A roll as claimed in claim 1 or claim 2 130nace.

4 GB2186595A 4 14. A process as claimed in any of claims 8 to 13 in which the running surface is melted by means of a metal inert gas welding torch.

15. A process of producing a roll, the roll and the process being substantially as described herein with reference to the foregoing Example.

Printed for Her Majesty's Stationery Office by Burgess & Son (Abingdon) Ltd, Dd 8991685, 1987. Published at The Patent Office, 25 Southampton Buildings, London, WC2A 1 AY, from which copies may be obtained.

1 1